Hessian matrix analysis applied to the HI4PI and the GALFA-H I observations. Top. Results obtained on a 3° × 20° × 1.29 km s⁻¹ tile in the HI4PI and the GALFA-H I observations, shown in the left and right panels, respectively. Bottom. Results obtained on two 3° × 20° × 0.184 km s⁻¹ tiles in the GALFA-H I observations. In each of the panels, the left-hand side map shows the H I emission convolved with a 2D Gaussian with the same diameter of the derivative kernel. The map in the center shows the second eigenvalues of the Hessian matrix, λ₋, which highlights the filamentary structure. The right-hand side map shows the orientation angle of the filamentary structure.